1. Field of the Invention
The present invention relates to a thermal transfer ink sheet that contains a polyvinyl acetal having a specific structure, and to an ink cartridge, a coating composition for dye layer of thermal transfer ink sheet, and a thermal transfer recording method.
2. Description of the Related Art
These days, in particular, a color image-forming material is the mainstream of an image-recording material; and concretely, inkjet-type recording materials, thermal transfer-type image recording materials, electrophotographic recording materials, transfer-type silver halide photosensitive materials, printing inks and recording pens are much used.
Thermal transfer recording includes a recording system where a thermal transfer material having a thermofusible ink layer formed on a base film (support) is heated with a thermal head to thereby melt the ink for recording on an image-receiving material, and a recording system where a thermal transfer material having a thermo-transferable dye-containing, dye-donating layer formed on a base film is heated with a thermal head to thereby thermally diffuse and transfer the dye onto an image-receiving material. In the latter thermal transfer system, the dye transfer rate may be varied by changing the energy to be applied to the thermal head, therefore facilitating gradation recording, and the system is especially advantageous for high-quality full-color recording.
In the thermal diffusion transfer recording system, a dye-containing thermal transfer sheet (hereinafter this may be referred to as “ink sheet”) and a thermal transfer image-receiving sheet (hereinafter this may be referred to as “image-receiving sheet”) are put one upon another, then the ink sheet is heated by a thermal head that is controlled for its heat generation by an electric signal, whereby the dye in the ink sheet is transferred onto the image-receiving sheet for image information recording thereon. In this, three colors of cyan, magenta and yellow are recorded as transferred one upon another, thereby attaining transfer recording of a color image having continuous color tone gradation.
The thermal transfer sheet contains an ink polymer that has a role of keeping a dye in the sheet, and heretofore, a polyester resin is used for the ink polymer. Polymer materials capable of forming high-density color images are reported in Japanese Patent No. 3263138 and JP-A 7-32742. However, with the recent tendency in the art toward much advanced speedup in printing, the resin could hardly satisfy both the requirement for speedup in printing and the requirement for photograph-level transfer sensitivity.